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jlechner |
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL" />
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 19." />
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<meta name="GENERATOR" content="vi and eight fingers" />
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<title>libstdc++-v3 HOWTO: Chapter 19: Diagnostics</title>
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<link rel="Start" href="../documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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title="Library Support" />
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title="General Utilities" />
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 19: Diagnostics</a></h1>
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<p>Chapter 19 deals with program diagnostics, such as exceptions
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and assertions. You know, all the things we wish weren't even
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necessary at all.
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</p>
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<hr />
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<h1>Contents</h1>
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<ul>
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<li><a href="#1">Adding data to exceptions</a></li>
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<li><a href="#2">Exception class hierarchy diagram</a></li>
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<li><a href="#3">Concept checkers -- <strong>new and improved!</strong></a></li>
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</ul>
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<hr />
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<h2><a name="1">Adding data to exceptions</a></h2>
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<p>The standard exception classes carry with them a single string as
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data (usually describing what went wrong or where the 'throw' took
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place). It's good to remember that you can add your own data to
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these exceptions when extending the hierarchy:
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</p>
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<pre>
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struct My_Exception : public std::runtime_error
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{
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public:
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My_Exception (const string& whatarg)
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: std::runtime_error(whatarg), e(errno), id(GetDataBaseID()) { }
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int errno_at_time_of_throw() const { return e; }
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DBID id_of_thing_that_threw() const { return id; }
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protected:
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int e;
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DBID id; // some user-defined type
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};
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</pre>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="2">Exception class hierarchy diagram</a></h2>
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<p>At one point we were going to make up a PDF of the exceptions
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hierarchy, akin to the one done for the I/O class hierarchy.
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Time was our enemy. Since then we've moved to Doxygen, which has
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the useful property of not sucking. Specifically, when the source
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code is changed, the diagrams are automatically brought up to date.
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For the old way, we had to update the diagrams separately.
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</p>
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<p>There are several links to the Doxygen-generated pages from
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<a href="../documentation.html">here</a>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<h2><a name="3">Concept checkers -- <strong>new and improved!</strong></a></h2>
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<p>Better taste! Less fat! Literally!</p>
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<p>In 1999, SGI added <em>concept checkers</em> to their implementation
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of the STL: code which checked the template parameters of
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instantiated pieces of the STL, in order to insure that the parameters
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being used met the requirements of the standard. For example,
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the Standard requires that types passed as template parameters to
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<code>vector</code> be "Assignable" (which means what you think
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it means). The checking was done during compilation, and none of
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the code was executed at runtime.
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</p>
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<p>Unfortunately, the size of the compiler files grew significantly
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as a result. The checking code itself was cumbersome. And bugs
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were found in it on more than one occasion.
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</p>
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<p>The primary author of the checking code, Jeremy Siek, had already
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started work on a replacement implementation. The new code has been
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formally reviewed and accepted into
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<a href="http://www.boost.org/libs/concept_check/concept_check.htm">the
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Boost libraries</a>, and we are pleased to incorporate it into the
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GNU C++ library.
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</p>
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<p>The new version imposes a much smaller space overhead on the generated
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object file. The checks are also cleaner and easier to read and
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understand.
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</p>
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<p>They are off by default for all versions of GCC from 3.0 to 3.4 (the
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latest release at the time of writing).
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They can be enabled at configure time with
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<a href="../configopts.html"><code>--enable-concept-checks</code></a>.
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You can enable them on a per-translation-unit basis with
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<code>#define _GLIBCXX_CONCEPT_CHECKS</code> for GCC 3.4 and higher
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(or with <code>#define _GLIBCPP_CONCEPT_CHECKS</code> for versions
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3.1, 3.2 and 3.3).
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr />
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<p class="fineprint"><em>
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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</em></p>
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</body>
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